Biotechnology firm, Dendreon, whose innovative treatment for prostate cancer was once predicted to become a blockbuster, filed for bankruptcy protection after sales of the drug fell way short of expectations and the firm could not pay a huge debt coming due.

Provenge, Dendron’s drug which was launched in the market in 2010, was the first approved cancer treatment that harnessed the patient’s own immune system to fight tumors.

Sales of Provenge were hampered by the drug’s high cost of $93,000 for a course of treatment, the complexity of customizing the therapy for each patient and an increase in competition from new pills for prostate cancer.

Such immunotherapy is now the biggest thing in cancer research, but attention is on approaches that are somewhat different from the one used by Dendreon.

Provenge’s sales once predicted to reach $4 billion a year are estimated to be approximately $300 million in 2014, up slightly from 2013. Dendreon said such a sales level would not be enough to allow the company to pay off $620 million in convertible debt, which is due in early 2016.

Seattle-based Dendreon added that it had reached an agreement on restructuring with holders representing about 84 percent of that debt.

Dendreon will try to sell its assets or itself. If it is unsuccessful, the note holders will convert their debt interest into equity and Dendreon will become a privately held company.

The chief executive of Dendreon, W. Thomas Amick said that whether the restructuring takes the form of a stand-alone recapitalization or a sale of the company or its assets, they are confident that this process will allow Provenge to remain commercially available to the patients and providers who have come to rely on this revolutionary personalized cancer immunotherapy.

Dendreon, stock was down around 70 percent, at about 30 cents per share. The shares reached around $50 on the day Provenge was approved in April 2010.